This invention relates to pneumatic conveying devices of the type wherein a vessel or "blow tank" is filled with solid particulate material to be conveyed and supplied with gaseous fluid under pressure to pressurize the tank. The invention also relates to pneumatic conveying apparatus using high pressure conveying air where material is conveyed at a high material to air ratio or in a "dense phase".
Prior to the present invention, it was generally known that it was desirable to control the air to material rate in the pneumatic conveying line to avoid material plugs and control the rate at which material is to be conveyed. Material plugs may occur due to a blockage in the line either mechanically induced or because of a build-up of material at pipe bends and other locations. If a high rate of material is continuously supplied to the conveying line during a build-up or line blockage, the pipe will eventually become further plugged and the system will have to be shut down to clear the plug. This is particularly a problem with dense phase conveying equipment.
Prior to the present invention, equipment known to the applicants controlled the material to conveying gas ratio by controlling the quantity of material being supplied to the conveying line. Such systems are shown for example in U.S. Pat. No. 3,403,941 issued Oct. 1, 1968 and U.S. Pat. No. 3,163,329 issued Dec. 29, 1964. While such apparatus functions adequately, the valves used for controlling the amount of material entrained into the gas conveying stream are exposed to direct contact by the material. Since the materials being conveyed may be abrasive, the valves tend to wear. Unlike other regulating mechanisms, this arrangement regulates the solids and air flow without being in direct contact with the abrasive solids.
Another system is shown in U.S. Pat. No. 3,599,832 where the material to air ratio in the conveying line is controlled by sensing the pressure in the conveying line downstream of the material feed point and utilizing this to control both the quantity of conveying air being supplied to the conveying line and the quantity of material being supplied to the conveying line. This system still relies on controlling the material to air ratio in the conveying line by controlling the quantity of material supplied to the conveying line.
According to the present invention the material conveying rate and the material to air ratio is controlled by regulating the quantity of gaseous fluid under pressure supplied to the inlet of the conveying line. This is done by comparing the pressure in the system to a predetermined set pressure and when this set pressure is exceeded, by passing conveying gas around the material feed point to the conveying line downstream of the material feed point. The conveying line between the point of introduction of material and the point at which the additional conveying air is supplied is substantially free of material flow control means during the material conveying cycle.
While it is known per se to bypass conveying air around the material feed point of a pneumatic transport system as shown by U.S. Pat. No. 1,889,163 issued Nov. 29, 1932, the system shown therein does not include the economies of the present invention.
A measure of pneumatic conveying system's efficiency is the weight of bulk solids conveyed per weight of air consumed. High pressure dense phase conveying systems are generally the most efficient systems with high solids to air ratios being achieved while discharging material from the tank. However, at the end of the discharge cycle (when the solids inventory is depleted from the tank) the tank still remains air-filled at the high conveying pressure. It is not uncommon to find more weight of air left in the tank after discharging the solids than that which flowed into the convey line alone with the solids during the discharge cycle.
Commonly, the trapped air is either vented through the convey line to purge the line free of solids thus preparing the line for the next discharge cycle or vented to the atmosphere through a vent valve and dust collector. Using this practice very little useful work is derived from this very significant quantity of air.
With the prior practice of venting large amounts of excess air to the conveying line, not only is a large quantity of air wasted, but also, this large volume of air containing particulate matter is travelling at high velocity which can cause damage to piping, valves in the line, and receiving tanks which are normally cushioned by a dense material content. In addition, such large quantities of air require larger pipelines and larger receiving vessels. According to the present invention, the air under pressure which remains in the tank is utilized to convey the final quantity of material from the vessel. Only a small volume of air need be vented to the conveying line after final clean out.